1. Field of the Invention
This invention relates to optical fiber preforms, and particularly to preforms that are prepared with multiple overclad tubes.
2. Discussion of the Known Art
Optical fibers for data and information transmission are typically produced by lowering one end of a glass fiber preform into the mouth of a vertical fiber draw furnace, and heating the preform as it descends through a hot zone inside the furnace. A drop of soft glass forms at the heated end of the preform, and an optical fiber is drawn from the soft drop. The preform may be assembled using a so-called rod-in-tube (RIT) technique.
In a RIT preform, a solid glass rod is supported axially inside a cylindrical glass overclad tube. The rod may be comprised only of core material, or possess a circumferential outer layer of cladding material. The overclad tube thus acts as a source of outer cladding on fibers that are drawn from the assembled rod and tube. The glass rod is referred to hereafter simply as a “core” rod, even though the rod typically possesses an outer layer of cladding material.
During fiber draw, the tube is heated until it softens and collapses about the rod, and the tube glass consolidates with the outer glass layer on the rod. An optical fiber with a relatively thick outer cladding layer may then be drawn from the consolidated rod and tube. This drawing process is sometimes referred to as overclad during draw or simply ODD. See also U.S. Pat. No. 6,460,378 (Oct. 8, 2002) entitled “Collapsing a Multitube Assembly and Subsequent Optical Fiber Drawing in the Same Furnace”, and commonly owned U.S. patent application Ser. No. 10/309,852 filed Dec. 4, 2002, entitled “Rod in Tube Optical Fiber Preform and Method”. All relevant portions of the mentioned '378 patent and the '852 application are incorporated by reference.
According to one embodiment disclosed in the '378 patent, a core rod is placed inside a first overclad tube, and a second overclad tube is arranged over the first overclad tube. The core rod and the two overclad tubes are heated under such conditions as to cause a partial collapse of the tubes at one end of the rod, thus forming a unitary multiple overclad preform. The one end of the preform is later set up for insertion into a vertical fiber draw furnace. An ODD fiber having a desired cladding to core mass ratio is then drawn inside the furnace as the tubes collapse further and consolidate with the core rod.
Because lengths of suitable core rods are typically much shorter than the uncut lengths of commercially available overclad tubes, it has been common practice to stack a sacrificial glass spacer rod above the core rod to obtain a “total” inner core length that matches the length of the overclad tube. Once all the material of the core rod is drawn into the core of an optical fiber, however, the remaining portion of the preform must be wasted. Accordingly, this procedure does not lend itself to a low cost and robust manufacturing process.
It is also known to weld a number of core rod segments axially in line end to end, to form a continuous long core rod. See, U.S. Pat. No. 4,195,980 (Apr. 1, 1980), and U.S. Pat. No. 4,407,667 (Oct. 4, 1983). This is a costly extra processing step, however, and negatively affects fiber quality in regions around the weld, i.e., added hydroxyl (OH) concentrations are produced by the welding heat source. See also, U.S. Pat. No. 6,434,975 (Aug. 20, 2002) disclosing a preform for producing a dispersion managed (DM) optical fiber, wherein the preform is assembled by selectively inserting a number of core rod tablets into a cladding glass tube, with adjacent tablets having different optical characteristics.
The use of larger preform sizes both in length and diameter, can yield cost benefits. As mentioned, the effective length of the core rod determines the useful length of the preform. But manufacturing long rods of core material, e.g., more than two meters in length, is difficult due to defects such as the formation of bubbles or deviations in optical properties beyond specified limits. Typically, only relatively short remnants will remain after defective portions of a single long core rod are cut away.
When preparing large size preforms of the overclad tube variety, satisfactory interfacial glass quality must be achieved. The interface between the outer circumference of the core rod and the inner circumference of the first overclad tube is critical, and must meet stringent material property requirements. For example, the concentration of hydroxyl (OH) ions or “water” greatly affects signal attenuation through so-called zero or low water peak (1383 nm) optical fiber. This requires that the first overclad tube be formed from an expensive high purity glass, as well as a large quantity of such glass if only one overclad tube is used for the preform. Other elements or ions that can act as impurities at the interface and, thus contribute to light signal attenuation through the drawn fiber include, without limitation, Chlorine (Cl), Al, Fe, Ca, Mg, K, Na, Li, Ni, Cr, Cu, Ti, V and Zn. See, R. H. Doremus, Glass Science (1973) at page 321, which is incorporated by reference.